Tarbosaurus was a very close relative to the Tyrannosaurus rex in size and also the massive skull which is 4 feet long. Tarbosaurus was a large dinosaur about 40 feet long and weighed 6 tons. It lived in Southern Mongolia in the late Cretaceous, 67 million years ago. It was one of the last surviving dinosaurs. New research by scientists suggests that Tarbosaurus is so close to T. rex, that there is still debate on whether or not to move the species T. baatar to the Tyrannosaurus genus.

It is believed that Tarbosaurus was, in fact, T. rex that migrated from North America to Asia through some sort of "land bridge" that connect the two continents during the Late Cretaceous period (around 70-66 million years ago).

Wikipedia has a more detailed and comprehensive article on Tarbosaurus